Center offset microwave frequency synthesizer

ABSTRACT

An improved microwave frequency signal source using a single frequency offset technique which increases the frequency range of an indirect frequency synthesizer to twice the highest operating frequency of the programmable digital frequency divider in the loop includes a voltage-controlled oscillator (VCO) operating within a predetermined microwave frequency band and phase-locked to a reference oscillator operating at a reference frequency below microwave frequencies. The offset loop signal is developed by heterodyning the voltage-controlled oscillator (VCO) output signal with a microwave signal whose frequency is located at the center of the predetermined microwave frequency band of the VCO to form a signal at an intermediate frequency (I.F.) within the frequency range of a programmable digital frequency divider.

BACKGROUND OF THE INVENTION

This invention pertains generally to microwave signal sources, and moreparticularly to a microwave signal source comprising a digital frequencysynthesizer using a single frequency offset technique with the frequencyrange of the signal source operating up to twice the highest operatingfrequency of a digital frequency divider in a phase-locked loop.

As is known in the art, microwave signal sources are a necessary elementof communication and radar systems. Digital frequency synthesizers offeran attractive approach to generation of microwave multifrequency signalsin applications not requiring continuous tuning capability. Digitalfrequency synthesizer architectures fall into two general categories.The first, known as direct frequency synthesizer architecture, isopen-loop and utilizes frequency multipliers, dividers and mixers as keycomponents. An advantage of such architecture is an extremely shortfrequency switching time. However, any known direct frequencysynthesizer architecture generates more spurious frequencies and hasmore elements than the second category known as "indirect frequencysynthesizer architecture."

Indirect frequency synthesizer architecture consists of digitalphase-locked loops where the frequency of a voltage-controlledoscillator (VCO) is selected by the frequency division ratio of aprogrammable digital frequency divider (hereinafter also referred to asa "digital divider"). In applications where the frequency of the VCOoutput signal exceeds the highest frequency capability of the digitaldivider, the VCO output signal is heterodyned to a signal of suitableselected lower frequency for use in a phase-locked loop, known as an"offset loop." An advantage of known indirect frequency synthesizerarchitecture is that lower spurious levels are experienced due to thelow-pass filter characteristics of phase-locked loops. A disadvantage ofknown indirect frequency synthesizer architecture is that a relativelylong frequency switching time is realized because of the characteristicsof known phase-locked loops. A further disadvantage of the indirectfrequency synthesizer architecture using offset loops is that thefrequency range of the output signal of the VCO is limited to thefrequency range of the digital divider.

A conventional indirect frequency synthesizer employing a digitalphase-lock loop consists essentially of a voltage-controlled oscillator(VCO) output signal phase-locked to a reference oscillator signal in asingle offset digital phase-lock loop. A signal from an offset generatoris used to heterodyne the full frequency range of the VCO output signalto lower intermediate frequencies (I.F.) within the frequency limits ofthe programmable digital frequency divider. The frequency of the offsetgenerator signal is either slightly below the lowest frequency of thesynthesizer output signal or slightly above the highest frequency of thesynthesizer output signal in order to produce I.F. signals over the fullfrequency range of the synthesizer. The frequency of the VCO outputsignal is determined by the division ratio of the digital divider, suchratio being set by a suitable digital word applied to a decoder. Theoutput signal of the digital divider and a reference frequency signalare connected to the two input ports of a digital phase/frequencydetector, the output signal of which is amplified in a loop amplifier,filtered by a loop filter and applied to the tuning port of the VCO. Thedigital word that commands a specific VCO frequency sets the frequencydivision ratio of the digital divider to a value that, for the specificVCO frequency, produces a signal with a frequency equal to the frequencyof the reference signal at the output of the digital divider. Followingthe application of a frequency select command, the frequency divisionratio of the digital divider changes, causing the loop to unlock. Undersuch conditions the phase/frequency detector acts as a discriminator andgenerates a voltage that, following amplification and filtering, tunesthe frequency of the VCO into a capture range of the loop. When thecapture range is reached, the phase/frequency detector takes on thecharacteristics of a phase detector and generates an error voltagerequired for phase-lock. Such a frequency synthesizer can generatesignals of equally-spaced frequencies, with the smallest spacingincrement equal to the reference frequency.

The frequency range that the frequency synthesizer can generate islimited by the highest operating frequency of the programmable digitalfrequency divider. In applications where wider frequency ranges have tobe generated this limitation is usually overcome by either usingmultiple frequency offset generators or by adding a higher frequencyfixed digital divider (prescaler) before the programmable digitalfrequency divider.

The disadvantage of the multiple frequency offset generator is in itsadded level of complexity because the frequency range of the synthesizersignal has to be partitioned into bands equal to the maximum operatingfrequency range of the digital divider signal, and one offset frequencyhas to be synthesized for every band.

The addition of a prescaler before the digital divider introducesseveral disadvantages. If the frequency division ratio of the prescaleris K, then the total frequency division ratio is increased by by thefactor K. In a second order loop (typically used in this type ofapplication) a K-fold increase in the frequency division ratio reducesthe highest realizable loop bandwidth by the factor √K and reduces thehighest realizable open-loop gain by the factor K (20 log K dB). Thereduced open loop gain magnitude causes a commensurate reduction in thecapability of the loop to degenerate FM noise. A reduction in therealizable loop bandwidth also increases the switching time whenfrequency is to be changed. Also, if the frequency division ratio of theprescaler is K, then the frequency of the reference oscillator signalalso has to be reduced by the factor K so that the phase/frequencydetector can operate with the same frequency signals at its two inputports. It is therefore desirable to increase the frequency range of theVCO output signal without increasing the frequency division ratio of theloop.

SUMMARY OF THE INVENTION

With the foregoing background of the invention in mind, it is a primaryobject of this invention to provide a microwave frequency signal sourceusing a single frequency offset technique to increase the frequencyrange of an indirect frequency synthesizer to twice the highestoperating frequency of a programmable digital frequency divider in theloop without increasing the frequency division ratio of the loop.

It is another object of this invention to increase the frequency rangeof an indirect frequency synthesizer without reducing the highestrealizable loop bandwidth.

It is another object of this invention to increase the frequency rangeof an indirect frequency synthesizer without reducing the maximumrealizable open loop gain and the FM noise degeneration capability ofthe loop.

The foregoing and other objects of this invention are generally attainedby providing a microwave frequency signal source using a singlefrequency offset technique wherein a voltage-controlled oscillatoroperating within a predetermined band of microwave frequencies isphase-locked, using a phase/frequency detector, to a referenceoscillator operating at a reference frequency below microwavefrequencies. An I.F. (intermediate frequency) signal is developed byheterodyning the output signal of the voltage-controlled oscillator witha siqnal from the output of an offset generator whose frequency islocated approximately at the center of the predetermined microwavefrequency band. The frequency of the output signal of the offset mixerused to heterodyne the signals out of the VCO and the offset generatoris within the frequency range of a programmable digital frequencydivider. Such divider is capable of dividing the input frequency by apredetermined division ratio to produce an offset loop signal requiredfor the phase/frequency detector to phase-lock the VCO output signal tothe reference signal provided by the reference oscillator. The change ofthe VCO frequency as related to the change of the I.F. frequency for VCOfrequencies below the offset frequency creates a relationship which isopposite to the relationship created when the VCO frequencies are abovethe offset frequency. For VCO frequencies below the offset frequency,the I.F. frequency decreases with increasing VCO frequency and for VCOfrequencies above the offset frequency, the I.F. frequency increaseswith increasing VCO frequency. Since a phase-lock loop cannot be stablewith both of these relationships, a switching means is used to switchthe polarity of the output signal of the phase/frequency detector toaccomplish each of the relationships. An initialization circuit ensuresthat when a frequency is selected above the offset frequency thefrequency of the VCO output signal frequency is above the offsetfrequency and when a frequency is selected below the offset frequencythe frequency of the VCO output signal is below the offset frequency.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing features of this invention, as well as the inventionitself, may be more fully understood from the following detaileddescription read together with the accompanying drawings, wherein:

FIG. 1 is a simplified block diagram of a microwave frequency signalsource according to this invention; and

FIG. 2 is a simplified block diagram of an alternative embodiment of amicrowave frequency signal source according to this invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to FIG. 1, a microwave frequency signal source 100 isshown to include a microwave, say an X-band, voltage-controlledoscillator (VCO 10). The output signal from VCO 10 is shown to becoupled, via a directional coupler 11, to an offset mixer 12. An offsetgenerator 14, here a crystal-controlled oscillator followed by afrequency multiplier capable of producing a microwave frequency signalapproximately at the center of the desired microwave frequency band, isconnected via offset switch 34 to the offset mixer 12 which is capableof heterodyning the output signal of VCO 10 and the output signal ofoffset generator 14 to produce an intermediate frequency (I.F.) signalwithin the input frequency range of a programmable digital frequencydivider (digital divider 20). The I.F. signal passes through low passfilter 16 which allows the desired band of frequencies to pass throughan I.F. amplifier 18 to raise the signal level to accommodate the inputof digital divider 20. Such divider is capable of dividing the frequencyof the signal out of the I.F. amplifier 18 by a predetermined divisionratio (N) as commanded by a decoder 22. Decoder 22 provides controlsignals, as required in response to a suitable digital word whichcorrelates to a specific microwave frequency of the output signal.Digital divider 20 is constructed in accordance with known art so as tobe capable of dividing the frequency of the I.F. signal by N, therebyproducing an output signal of the same frequency as the referenceoscillator 24. The output signal of the digital frequency divider 20 isapplied to an input of phase/frequency detector 26.

Digressing briefly here now for a moment, it will be noted by those ofskill in the art that this introduces an ambiguity because every I.F.frequency has two corresponding VCO frequencies. This ambiguity can beeliminated by the addition of a VCO frequency initialization circuit 40.When a frequency select command is applied to the decoder 22, a signalfrom the decoder 22 causes the initialization circuit 40 to apply anappropriate D.C. voltage signal to loop amplifier 30. If the selectedfrequency is below the offset frequency, the initialization circuit 40positions the frequency of the VCO output signal below the frequency ofthe offset generator. If the selected frequency is above the offsetfrequency, the initialization circuit 40 positions the frequency of theVCO output signal above the frequency of the offset generator. Thistechnique eliminates frequency ambiguity because the selected frequencyand an initialization frequency are always on the same side of theoffset frequency.

It will also be appreciated by those of skill in the art that for eachintermediate frequency two relationships exist between the VCO frequencyand the I.F. frequency. For VCO frequencies below the offset frequencythe I.F. frequency decreases with increasing VCO frequency; for VCOfrequencies above the offset frequency the I.F. frequency increases withincreasing VCO frequency. If the loop (not numbered) is configured toexhibit negative feedback (stability) when the VCO frequency is greaterthan the offset frequency, then the loop (not numbered) exhibitspositive feedback when the VCO frequency is less than the offsetfrequency. To correct for the latter condition negative feedback isrestored by switching (interchanging) the two outputs of thephase/frequency detector 26 which reverses the polarity of an outputsignal (error voltage) of a loop amplifier 30.

The phase/frequency detector 26 has two inputs. The first input is froma reference oscillator 24, here a crystal-controlled oscillatoroperating at 10 MHz, which is capable of producing a reference frequencysignal. The second input is from the digital divider 20. Followingfrequency initialization the phase/frequency detector 26 acts as adiscriminator and develops an output signal that changes the frequencyof the VCO output signal from an initialization frequency to a capturerange of the loop about the selected frequency. When the capture rangeis reached, the phase/frequency detector 26 takes on the characteristicsof a phase detector and generates a control signal required forphase-lock. The output of phase/frequency detector 26 is connected vialoop switch 28 to loop amplifier 30 which increases the signal level andpasses through loop filter 32 before being applied to a tuning controlinput of VCO 10.

When the selected frequency is above the offset frequency, the I.F.frequency increases with increasing VCO frequency and when the selectedfrequency is below the offset frequency the I.F. frequency decreaseswith increasing VCO frequency. When a frequency is selected above theoffset frequency, decoder 22 sends a control signal to loop switch 28which causes loop switch 28 to connect the output signal ofphase/frequency detector 26 to loop amplifier 30 such that an increasein the I.F. frequency causes an increase in the frequency of the VCO 10output signal. When the selected frequency is below the offsetfrequency, decoder 22 sends a control signal to loop switch 28 whichcauses loop switch 28 to connect the output signal of phase/frequencydetector 26 to loop amplifier 30 such that an increase in the I.F.frequency causes the frequency of the VCO 10 output signal to decrease.

It will be appreciated by those of skill in the art that by using anoffset frequency in the center of the desired band the VCO 10 cannotgenerate a signal with a frequency equal to the offset frequency. Thiscan be solved by using the output signal of offset generator 14 as theoutput signal of microwave frequency signal source 100. The outputsignal of offset generator 14 is switched, via offset switch 34, toeither an input of offset mixer 12 or to the input of offset amplifier38 as commanded by decoder 22. When the selected frequency is not theoffset frequency, offset switch 34 is in position to connect the outputsignal of offset generator 14 to an input of offset mixer 12. When theselected frequency is equal to the offset frequency a control signalfrom decoder 22 places offset switch 34 in position to connect theoutput signal of offset generator 14 to the input of offset amplifier38. Offset amplifier 38 is capable of amplifying the output signal ofoffset generator 14 to a level equal to the level of the VCO 10 outputsignal so the next stage (not shown) will receive the same signal levelwhen selected. Output switch 36, also commanded from decoder 22, placeseither the VCO 10 output signal or the output signal of offset amplifier38 at the output of the microwave frequency signal source 100.

Referring now to FIGS. 1 and 2, a second embodiment according to theinvention is shown wherein like references indicate like parts asdescribed in the first embodiment but where switching circuit 50 isreplaced by second switching circuit 60. The output signal from VCO 10is coupled, via directional coupler 11, to image rejection mixer 52.Offset generator 14 is connected to image rejection mixer 52 via offsetswitch 34. The operation of the image rejection mixer 52 is such thatany signal out of the VCO with a frequency above the offset frequencyproduces an I.F. signal directed to a first output port (not numbered)while any signal out of the VCO with a frequency below the offsetfrequency produces an I.F. signal directed to a second output port.Mixer switch 54, commanded by decoder 22, selects one of the two outputsof image rejection mixer 52, depending on whether the selected frequencyis below or above the offset frequency and connects the signal to theinput of low pass filter 16. If the selected frequency is above theoffset frequency then mixer switch 54 selects an output of imagerejection mixer 52 such that an increase of the I.F. frequency causes anincrease of the frequency of the VCO output signal. If the selectedfrequency is below the offset frequency then mixer switch 54 selects theother output of image rejection mixer 52 such that an increase of theI.F. frequency causes a decrease of the frequency of the VCO outputsignal. In this second embodiment loop switch 28 is not used; therefore,the output signal of phase/frequency detector 26 is fed directly intothe input of loop amplifier 30.

As previously explained, to ensure the selected frequency and theinitialization frequency are on the same side of the offset frequency,initialization circuit 40, commanded by decoder 22, provides a controlD.C. voltage to loop amplifier 30 to drive the frequency of the VCOoutput signal above or below the offset frequency, depending on thefrequency selected. As an alternative embodiment, VCO 10 could have acoarse tune input as a second input and an alternative initializationcircuit (not shown) could be used to provide a control signal to VCO 10to ensure the frequency of the VCO 10 output signal is on the correctside of the offset frequency at initialization. As in the firstembodiment, offset switch 34, offset amplifier 38 and output switch 36are capable of switching the output of the offset generator 14 to theoutput of the microwave frequency signal source 100 when the frequencyselected is equal to the offset frequency. Otherwise, offset switch 34places the output signal from offset generator 14 at an input of imagerejection mixer 52 and output switch 36 places the output of VCO 10 atthe output of microwave frequency signal source 100.

Having described a preferred embodiment of this invention, it will nowbe apparent to one of skill in the art that many changes may be madewithout departing from our inventive concepts. For example, thefrequencies at which the oscillators operate and the band of microwavefrequencies over which the VCO is operative as described may be changedas required. It is felt, therefore, that this invention should not berestricted to its disclosed embodiment, but rather should be limitedonly by the spirit and scope of the appended claims.

What is claimed is:
 1. In the operation of an improved microwavefrequency signal source using an indirect frequency synthesizerarchitecture including a voltage-controlled oscillator (VCO), responsiveto a control signal having either one of two polarities which produces amicrowave frequency output signal within a predetermined band ofmicrowave frequencies, the method comprising the steps of:(a) producingan offset frequency signal having a frequency approximately at themiddle of a predetermined band of microwave frequencies and heterodyningthe offset frequency signal with the output signal of the VCO, therebyforming an intermediate frequency (I.F.) signal having a frequencyindicative of the magnitude of the difference between the frequency ofthe offset frequency signal and the output signal of the VCO; (b)dividing the I.F. signal by a predetermined division ratio producing afrequency divider output signal having a frequency lower than thefrequency of the I.F. signal; (c) forming a first portion of the controlsignal with an amplitude in accordance with the frequency divider outputsignal; and (d) changing the polarity of the first portion of thecontrol signal in accordance with the frequency of the output signal ofthe VCO relative to the frequency of the offset frequency signal toproduce the control signal to be applied to the VCO.
 2. An improvedmicrowave frequency signal source, such source comprising:(a) a first,second and third oscillating means where the first oscillating means isa voltagecontrolled oscillator (VCO) for producing a microwave frequencysignal anywhere within a predetermined band of such signals in responseto an applied control signal having either one of two polarities, wherethe second oscillating means is an offset generator capable of producinga microwave frequency signal about the middle of the predetermined band,and where the third oscillating means is a crystal-controlled oscillatorcapable of producing a reference signal of frequency lower than thefrequency of microwave signals; (b) mixing means, responsive to theoutput signals of the first and second oscillating means, for producingan intermediate frequency signal; (c) dividing means, responsive to theintermediate frequency signal out of the mixing means for dividing thefrequency of such signal by a predetermined division ratio to produce anoutput signal at a frequency approximating the frequency of the signalproduced by the third oscillating means, the predetermined divisionratio being commanded in response to a digital word indicative of aspecific microwave frequency of for the output signal of the microwavefrequency signal source; (d) detection means, responsive to the signalproduced by the third oscillating means and the output signal of thedividing means, for producing the control signal to control thefrequency of the first oscillating means output signal; and (e) firstswitching means, actuable in accordance with whether the frequency ofthe first oscillating means is selected to be above or below thefrequency of the second oscillating means, for changing the polarity ofthe control signal, whereby the frequency of the first oscillating meansis changed until the control signal is nulled.
 3. A microwave frequencysignal source as in claim 2 wherein:(a) the first switching meanscomprises a switch responsive to a second control signal that controlsthe polarity of the control signal out of the detection means; (b)second switching means comprising: initialization means responsive to athird control signal capable of driving the frequency of the VCO outputsignal either above or below the frequency of the output signal of theoffset generator; and (c) third switching means comprising: output meansresponsive to a fourth control signal that switches, selectively, theoutput signal of the offset generator or the output signal of the VCO tothe output of the microwave frequency signal source.
 4. A microwavefrequency signal source as in claim 2 wherein the mixing meanscomprises:(a) an image rejection mixer capable of heterodyning theoutput signal of the offset generator with the output signal of thevoltage-controlled oscillator to produce two intermediate frequencycontrol signals such that VCO output signals with a frequency above theoffset frequency produce an I.F. signal directed to a first output portwhile VCO output signals with a frequency below the offset frequencyproduce an I.F. signal directed to a second output port; (b) firstswitching means comprising a switch responsive to a second controlsignal capable of selecting one of the two intermediate frequencycontrol signals; (c) second switching means comprising: initializationmeans responsive to a third control signal capable of driving thefrequency of the VCO output signal either above or below the frequencyof the offset generator; and (d) third switching means comprising:output means responsive to a fourth control signal that switches,selectively, the output signal of the offset generator or the outputsignal of the VCO to the output of the microwave frequency signalsource.